Oil from oil bearing earth formations is produced by the inherent formation pressure. In many cases, however, the oil bearing formation lacks sufficient inherent pressure to force the oil from the formation upwardly through a string of production tubing and to the surface where it will be transported from a wellhead structure by flow lines. When the pressure of the production zone has been reduced by continued withdrawal, a time arrives when the well will not flow from its reservoir energy. When this occurs, one method of continuing production is to provide mechanical pumping operations. Another popular method for achieving production from wells that no longer are capable of natural flow is by the gas lift method whereby gas is injected into the annulus between the production tubing and the casing under controlled conditions.
The concept of using gas as a means of artificial lift of well fluids evolved in the late 1700's. The early methods were designed primarily for continuous flow operations. Continuous flow gas lift has been defined as a means of artificial lift where gas is continuously injected from the surface down the annulus defined between the tubing and the casing of a well, through a gas lift valve between the annulus and the tubing and up the tubing string. The gas mixes with and aerates the fluids in the tubing string thereby providing a lifting force for lifting the fluids to the surface. Gas was traditionally injected either around the bottom or through a piece of equipment commonly called a foot piece.
A technology developed which provided for selective injection of gas into the tubing string through gas lift valves which are well known in the art. Intermittent gas lift is a means of artificial lift where a slug or column of liquid is allowed to accumulate in the tubing string, whereupon gases are injected through a gas lift valve underneath the liquid slug to propel it to the surface in the form of a slug. U.S. Pat. No. 4,392,532 reveals such a system. A wide variety of gas lift valves have been designed specifically for intermittent lift.
Spacing and other characteristics of the gas lift valves must be established in accordance with the criteria of the particular well involved in order to achieve production at the maximum rate that is producible from the formation involved. For the reason that no two wells are exactly alike and may involve differences in such parameters as the height of the static liquid column within the well, the static gradient of the liquid fluid, i.e., liquid between the valves, and geothermal temperature, it is virtually required that each gas lift system for independent wells be separately calculated to achieve optimum production.
With both continuous and intermittent gas lift it is required that substantial volumes of gas at substantial pressures be produced at the surface of the well to achieve desired results. In addition, numerous valves are required in known systems to provide suitable pressures at the points where the gas is introduced into the tubing string. Because substantial pressures must be produced at the surface to force the substantial volumes of gas down the well for gas lift, the equipment in the form of compressors, tanks, conduits, valves and the like which is required to handle the gas is substantial and expensive.
The high pressure components of the equipment require careful maintenance to avoid expensive or dangerous failures and consume substantial quantities of energy.
Gas lift systems are usually applied to wells that produce from water driven reservoirs, or in reservoirs, which, although incapable of natural flow will have sufficient pressure throughout their life to provide the submergence required for efficient lift.
The overall efficiency of a gas lift system producing from a well with a strong water drive can be quite high. In present day systems, however, designers, faced with the necessity of unloading to the deeper depths, say to 4,000 or 5,000 feet, have resorted to use of minimum gradient curves which provide for inefficient operation. Observed efficiencies in some of these wells have ranged from seven to eleven percent.